Oils and fats consist of complex mixtures of triacylglycerols (TAGs), diacylglycerols (DAGs), free fatty acids and other minor components. The crystallisation of these mixtures depends on the characteristics of the TAGs (structure, chain length, saturation compared with unsaturation, and the like) and the interaction of these TAGs with each other. Regarding the presence of DAGs, previous studies have shown that they have significant effect on the physical properties of oils and fats. These vary from rate of crystallisation, polymorphism changes, melting point, crystal size and habits (Siew, 2001).
In most oils that are extracted from oilseeds, the effect of DAGs is less pronounced, as DAGs are only present in small quantities. In primarily palm oil and olive oil, which are oils containing high natural amounts of DAGs, however, the quality of these oils suffers if DAGs are present therein.
Palm oil obtained from oil palm (Elaeis guineensis) is commercially important edible oil. Palm oil has been a prominent fat and oil resource for the food industry due to several advantageous properties, such as high productivity, low price, high thermal and oxidative stability and plasticity at room temperature. In addition, compared with other vegetable oils, palm oil is a rich source of the anti-oxidant vitamin E.
A typical chemical composition of refined palm oil is about 93% triglycerides, 6% diglycerides and 1% monoglycerides (MAGs) (Okiy, 1977).
When palm oil crystallizes, a complex 3-dimensional network of the present components is formed. In the theory it is described that the bigger the diversity of the building blocks (TAGs, DAGs and MAGs) in the network, the more complicated the network will be and the slower the crystallization will happen (Jacobsberg & Ho, 1976). This theory was confirmed by Drozdowski (1994). Furthermore, his studies showed that the more the fatty acid composition variation in the triacylglycerol molecule, the more difficult was the transition between the different crystal phases.
As previously mentioned, a high content of diglycerides in palm oil affects its crystallization properties (Okiy et al., 1978, Okiy, 1978).
The presence of diglycerides in such oils is disadvantageous. In particular, diglycerides in edible oils (in particular palm oil) can lead to a low quality oil.
The problems relating to the diglyceride content in palm oil and other edible oils and fat have been the subject of many studies and different solutions to attempt to overcome the problem of too much diglyceride can be found in the literature.
The Japanese enzyme producer Amano on their home page (Amano Enzyme Inc., 2004), recommend an enzymatic process to remove diglyceride in fats and oils. This process is based on the use of an enzyme LIPASE G “AMANO” 50 which is able to degrade diglycerides to free fatty acids and glycerol. This enzyme is a diglyceride (DAG) and/or monoglyceride (MAG) hydrolyzing. The free fatty acids produced are removed by vacuum distillation or fractional crystallisation.
EP 0 558 112 describes a process for the enzymatic hydrolysis of residual diglycerides in triglyceride preparations in emulsions. The process is based on the hydrolysis of diglyceride with Lipase G from Amano, Japan (supra). The process was enhanced by making the enzymatic reaction in an emulsion for the degradation of diglyceride to fatty acids and glycerol. The water phase is separated after reaction and the enzyme is partly reused.
JP 62061590 teaches a hard butter containing low amounts of diglyceride, which is manufactured by treating oils or fats with a partially glyceride-specific enzyme (e.g., a lipase) in the presence of a catalytic amount of water and by a lipase that is a 1,3-specific enzyme in the presence of fatty acids, fatty acid esters, or other glyceride oils or fats. The product is hard butter especially suitable for use as a cacao butter substitute. Thus, lipase G and Rhizopus deremer lipase (1,3-specific enzyme) were mixed with diatomaceous earth and granulated. The granules was mixed with palm medium melting point fraction (5.7% diglyceride, acid value 0.25) and water (10% with regard to the partial glyceride-specific enzyme). The mixture was stirred at room temp. for 1 h, and the enzymes and water removed to give a hard butter containing 1.2% diglyceride (acid value 10.5).
The prior art thus teaches ways to reduce or remove the content of diglyceride in palm oil and other edible oils by enzymatic reactions. These processes rely on the hydrolysis of diglyceride with a specific diglyceride hydrolysing lipase during formation of free fatty acids and glycerol. The free fatty acids can then be removed by means of different processes like vacuum distillation or fractionation.
The disadvantage of using a specific diglyceride hydrolysing enzyme is the disadvantageous formation of free fatty acids. These free fatty acids have to be removed from the palm oil. Thus, the formation of free fatty acids is often considered as loss of product.
To overcome the problems with the removal of free fatty acid and the loss of product caused by the free fatty acid formation we have found a new method to overcome the problems with high diglyceride content in palm oil and other vegetable oils.
Enzymatic removal of diglycerides from palm oil has been taught by use of lipases, which are typically 1,3 specific triacylglycerol hydrolyzing enzymes (E.C. 3.1.1.3) (for example see JP 62061590 or EP 0 652 289). WO00/05396 teaches inter alia treatment of a food material which may comprise glycerol with a lipase to effect glycerolysis in a low water environment.
However, both 1,3 specific triacylglycerol hydrolyzing enzymes (lipases) and DAG/MAG hydrolyzing enzymes result in a significant increase in free fatty acid in the oil, and also result in the hydrolysis of monoglycerides.
However, in some vegetable oils for some applications for example it may be desirable to increase the monoglyceride content of the oil as this provides emulsifier functionality. Thus, in one aspect it is preferable to reduce diglyceride content without decreasing the monoglyceride content. In another aspects it may preferably to reduce both the diglyceride and monoglyceride content.
Lipase enzymes can also result in a detrimental increase in DAG due to the hydrolysis of triacylglycerol (TAG), the bulk lipid present in food oils.